Design technology of photovoltaic inverter without isolation transformer

Central topics:

• Basic Design of Grid-connected Photovoltaic Inverter
• Key technologies for photovoltaic inverters without isolation transformers
• Analysis of leakage current in normal operation of photovoltaic inverter
• Fault Current Analysis in Photovoltaic Panels
• Analysis of DC Component Entering AC Grid

Basic design of grid-connected photovoltaic inverters

Regardless of the technology used, the basic design of the inverter is clear and very similar. At its core is the process of converting a DC voltage (PV module) into an AC voltage (grid-connectable). During the conversion process, the positive and negative connections of the DC current are constantly switched, resulting in an AC current with a changing direction. Therefore, the key component of the inverter is the bridge switch (transistor element, see Figure 1a)), which is connected to the incoming DC power supply on one side and to the AC grid on the other side. During operation, only two opposing switches can be closed at the same time.

If the switching speed of this switching bridge is set to the same grid frequency, it is theoretically possible to connect the output side of the bridge to the grid. However, since the output current is a square wave with no change in intensity, an inductor with an iron core is installed at the output end to control the output current to a sine wave shape. The opening of the bridge is carried out in a pulse process, resulting in a small current component associated with the pulse. Such a current component can control the current in the inductor. The frequency of the pulse is generally 20KHz, so that a 50Hz current can be formed, as shown in Figure 1b).

For photovoltaic inverters, there is another very important device that cannot be omitted: the capacitor at the input end, as shown in Figure 1c). The function of the capacitor is to store electrical energy, ensure that the current from the power generation side is continuously and consistently supplied to the bridge switch, and enter the power grid through the bridge that changes synchronously with the power grid frequency. Only when the capacity of the input capacitor is large enough can the continuous and normal operation of the photovoltaic power generation system be guaranteed.

Figure 1: Basic design of a photovoltaic inverter

Figure 2 describes the basic functions of an inverter that can be used for direct grid connection. However, in practical applications, the input voltage range has certain limitations. For grid-connected power generation applications, the input voltage must be higher than the peak voltage of the grid at all times. When the effective value of the grid voltage is 250V, the minimum voltage on the power source side that meets the normal grid connection requirements should be 354V.

Figure 2: Overview of the most commonly used inverter circuit diagrams

Different from the basic design of standard inverters, direct grid-connected inverters have many ways to adjust or increase the input voltage range. Commonly used inverter technical solutions and structures are different:

The inverter topologies mentioned above differ not only in terms of electrical isolation, but also in terms of achievable efficiency, voltage dependency, etc. Therefore, there is no uniform formula to define which inverter design is the best design, and the user must take into account the specific inverter characteristics used.

Key technologies for photovoltaic inverters without isolation transformers

At present, as long as the photovoltaic power station is designed properly, it can be operated economically. Transformerless inverters that are directly connected to the grid are increasingly valued due to their low cost and high efficiency. However, this technology is still considered "problematic". This will be examined and explained below.

The transformer converts electrical energy into magnetic energy and then converts magnetic energy into electrical energy. The electrical isolation device installed between the input and output leads to energy losses of 1% or even up to 2%. Therefore, transformerless inverters operate more efficiently than transformer inverters. This technology has many other advantages, such as low material consumption and low weight.

In short, transformerless inverters are relatively small in size, light in weight, and cheaper in price, which is more advantageous than transformer inverters in many aspects. Although electrical isolation is not required for the operation and safety of photovoltaic power plants, the following aspects should be considered when designing inverters that are directly connected to the grid.

Figure 5: Same appearance, different internal circuits: efficiency characteristics of two Sunny Boy models, transformer type and transformerless type.

Leakage current in normal operation

When the voltage from the photovoltaic module is converted at a high frequency (20kHz), the high-frequency voltage should be equivalent to the peak value of the grid voltage; these voltages are considered interference inside the inverter, and the filter can block these interferences and prevent them from entering the grid. However, in theory, it is impossible to absolutely prevent the DC component from the power supply side from entering the AC grid.

In this way, depending on the inverter structure used, there will be different DC voltage components to the ground in the AC output. If there is an AC voltage between the solar cell group and/or its terminal to the ground, a "leakage current" will be generated, which will flow to the ground point of the battery group through the parasitic capacitance.

Figure 6: Sunny Boy 2100TL inverter photovoltaic battery group to ground voltage

Figure 7: Sunny Boy 5000TL HC multi-string inverter photovoltaic battery group to ground voltage

Let's take the Sunny Boy 2100TL and Sunny Boy 5000TL HC inverters as examples. As shown in the figure above. The operation of these two inverters will generate time-dependent potentials in their electronic parts, and their PV modules have different voltages to ground. Sunny Boy 2100TL uses an H-bridge structure, and the voltage applied to the PV modules is half the effective value of the grid voltage. The

multi-string inverter SB5000TL HC uses a capacitor half-bridge structure. The neutral line of the bridge is directly connected to the neutral line of the grid. The result is that the voltage to ground is only a low voltage value of 50Hz, and its component is only a small part of the grid voltage, which is only equivalent to the voltage ripple in the transformer topology. In addition to

the consideration of grid voltage boost, the magnitude of the leakage current also depends on the magnitude of the parasitic capacitance of the PV module, which is related to the battery area and the distance between the module and the frame. Therefore, regarding the leakage current, the structure of the inverter and the size of the PV module should be carefully considered when designing the system. The larger the area and the smaller the distance between the battery and the frame of the PV module, the greater the leakage current generated. The leakage current value of frameless photovoltaic modules is very low. However, amorphous cells mounted on stainless steel foil will generate a large leakage current.

External conditions also affect the leakage current, so it is inevitable that there will be certain fluctuations. If sediment or cleaning fluid wets the photovoltaic module, the leakage current will increase; the electronic material components in these liquids shorten the distance between cells and cause the leakage current to increase.

In short, the leakage current of photovoltaic modules during operation (under normal circumstances) depends on many operating conditions and there is no fixed value to measure it. Taking H-bridge inverters (such as Sunny Boy 2100TL) as an example, the leakage current value of photovoltaic modules during operation is in the range of 1-30mA/KWp.

Fault current in photovoltaic modules

In photovoltaic power stations for grid-connected applications, only photovoltaic modules with reliable insulation between the cells and the frame can be used. The modules must have double or super strong insulation measures, and the system pressure resistance of the photovoltaic modules must be fully considered to ensure that the surface of the modules can be touched even when the photovoltaic system is in operation without causing danger. At present, all photovoltaic modules can reach level II protection, and there are no strict restrictions when choosing.

As mentioned above, for transformerless inverters, the voltage on the PV module during operation can be the synchronous voltage value of the AC grid superimposed. When the surface of the module is touched, a fault current to the ground may be generated. If the insulation of the module is good enough, it is generally difficult to generate such a current. However, the intensity of the fault current discharge will increase with changes in some conditions, such as the shortening of the distance between the photovoltaic cells (in this case, the thickness of the transparent glass or plastic plate is reduced), the increase in the contact area, etc. For example: because the liquid used to clean the photovoltaic module contains conductive substances, the conductive area will be expanded, resulting in unexpected fault current. In this case, although the dangerous current cannot be detected in advance, it will cause certain dangers if an accident occurs. In order to avoid the safety hazards caused by this (such as suddenly falling from a ladder, etc.) and to avoid danger, when building a photovoltaic grid-connected power generation system, users should follow the following steps:

1) Connect the frame and other conductive parts of the photovoltaic module to the ground wire
2) Disconnect the inverter from the grid when maintaining the system or cleaning the photovoltaic module

With these protective measures, personnel safety can be fully guaranteed. Precision-designed transformerless inverters have additional protection, even if the safety standards required by electrically isolated inverters are exceeded, there is no need to worry about safety issues.

In this type of inverter, the DC or AC leakage current that may be generated by the components is continuously monitored. Once a fault current (greater than 30mA) is generated, the inverter is immediately disconnected from the grid. However, in real applications, monitoring fault currents is more complicated than simply monitoring the leakage current size. The leakage current changes at any time when the system is running, and the current value cannot be known before the grid is connected. Therefore, before each inverter is connected to the grid, the insulation resistance of the photovoltaic module is tested. Only when the insulation resistance exceeds the required resistance value (greater than 1M ohm) can it be proved that there is no fault current injected into the grid, and the grid can be connected at this time. Therefore, the identification of fault current is not only obtained by monitoring the increase of leakage current, but also by measuring the rate of change of current. All fault current monitoring devices must have a leakage current detection function (dual), and each monitoring system must be able to independently identify the fault current. In this way, personal safety will be more guaranteed. RCD protection rarely or never requires manual testing after commissioning, but the above protection measures are far more effective than general RCD protection.

DC component entering the AC grid

Direct connection to the grid usually results in DC power entering the AC grid directly. This DC component affects the normal operation of the equipment on the grid (local grid transformer) and the working characteristics of the RCD. At the same time, it causes internal friction in the transformer of the electrical appliances connected to the grid, resulting in magnetic saturation, which is not the required use environment for the electrical appliances. Although this situation does not necessarily damage the equipment, it can trigger the activation of the protection equipment in the grid to prevent DC components. Therefore, in theory, grid-connected inverters are equipped with preventive measures to prevent DC power from entering the grid (entering the grid through 50Hz transformers or capacitors).

It is also very important that the ability of the inverter to feed DC power into the grid does not only depend on the presence of an isolation transformer. In combination with a capacitor, the transformer can only transmit power under electrical isolation. In fact, we are concerned with the ability of the electrical components in the circuit to input DC current into the grid. For high-frequency transformer inverters directly connected to the grid, ordinary inverter bridges can feed DC current to the grid with or without a transformer.

For SMA inverters, the capacitor is part of the bridge. The transformer of transformer-type inverters is set on the grid side of the bridge, so that only AC current can be supplied to the grid (such as Sunny Boy 5000TLHC and all transformer-type inverters).

Even if the inverter bridge fails, it is impossible to continue to feed DC current to the grid. The reason is that the two bipolar relays connected in series in the inverter will cut off the connection with the grid in this case. This solution is applied to all SMA transformerless inverters. Assuming that the relay fails, the short circuit of the bridge will cause an overcurrent. The overload protection (overload switch) in the inverter will still start and cut off the connection with the grid.

Conclusion

PV power plants using transformerless inverters have the advantage of high power generation. In terms of safety, they are completely comparable to power plants with physical electrical isolation devices. Due to the use of a complete personnel protection device inside, the drive of the device is completed by a system with automatic leakage current monitoring function, and the protection capacity is more ideal. When designing a photovoltaic power plant, the following points should be fully considered:

• Choose photovoltaic modules and cables with good insulation (level II protection)
• Connect the PV panels and/or PV panel frames to the ground
• Select transformerless inverter with complete fault current detection and monitoring
• Note that when the capacitor is connected to the grid, the DC component fed into the grid needs to be monitored.
• When fault current detection is required at the power supply contact, attention should be paid to the leakage current of the component during operation (such as setting the leakage current monitoring value to 100mA or higher)
• When repairing a photovoltaic power station, disconnect the inverter.

Since the investment recovery period of a photovoltaic power station mainly depends on the power generation, the conversion efficiency of the inverter is particularly important. In view of the advantages of the SMA system, transformerless inverters will occupy a more important position in the competition of the photovoltaic market.